Aqueous dispersions based on vinyl acetate with further comonomers are prepared traditionally in a batch process. This procedure allows utmost flexibility for a broad product portfolio with small to medium-scale products; medium-scale here refers to a volume of up to 10 000 T per annum. For higher-volume products, continuous processes would also be considered. A disadvantage associated with the continuous process is the high output rate of the plant, which, while in principle desirable, makes this process unattractive for products having relatively small sales volumes, since the quantities produced are too great in too short a time and have to be stored. On the other hand, of course, even for relatively small-scale products, there is a need to increase productivity in the batch process, since the batch process is significantly less efficient than a continuous process.
The terms are defined here once again for clarification: Batch processes are discontinuous processes in which all of the reactants (apart from the initiators, or only portions of the initiators) are included in the initial charge, and only the initiators, or portions of the initiators, are metered in.
In the discontinuous semibatch process, the reactants are not included in their entirety in the initial charge; instead, also monomers and aqueous solutions/emulsions of protective colloids and/or emulsifiers are at least partly metered in.
In the continuous process, all of the reactants, after a starting phase (generally a batch operation), are metered continuously over a long time (several days to weeks) into the reactor or into a reactor cascade, and the product is withdrawn continuously.
Frequently in everyday language no distinction is made between batch and semibatch processes, and caution is therefore called for when discussing them. Moreover, frequently, the term “semicontinuous” is used in the literature for the process identified in this specification as “semibatch”. The key characteristic of (semi)batch operation is that it is ended after complete filling of the reactor has been reached, and the reactor is emptied, and a new operation is started. The fundamental principle of a continuous operation, frequently in stirred tank cascades, is that the reaction components are fed continuously into the reactor and the polymerization product is withdrawn continuously at the same rate.
The continuous polymerization of vinyl acetate and ethylene is known and is widely described in the patent literature:
DE-A 2456576 describes a process for the continuous polymerization of vinyl acetate-ethylene dispersions in a pressure reactor with downstream reaction tube. To prevent wall deposits and to ensure a uniform polymerization profile, the use is recommended of a redox initiator mixture containing a 3- to 10-fold excess of reducing agent.
EP 1174445 A1 describes a seed latex process for the continuous polymerization of vinyl ester-ethylene copolymers, in which the end product itself is used as a seed latex. Relatively coarse polymer particles, having a weight-average diameter of more than 1000 nm, are obtained.
EP 1323752 A1 describes a process for the continuous polymerization of protective-colloid-stabilized polymers on the basis of vinyl esters and ethylene. With polyvinyl alcohol used as protective colloid, dispersions having a weight-average diameter Dw of 800 to 1400 nm are obtained, the protective colloid being metered into both reactors.
A semicontinuous process combines the two fundamental principles of batch operation and of continuous operation. The reactor is started in discontinuous batch (or semibatch) operation until complete filling is reached. At the time point at which batch operation is ended on account of the attainment of complete filling (usually this is done ahead of this time point, in order to prevent what is termed liquid pressure), continuous operation begins, with material being supplied to and withdrawn from the reactor simultaneously. This second phase may in principle last for as long as is desired, but will in practice be guided by the volumes of the downstream storage containers.
A semicontinuous polymerization process is described in U.S. Pat. No. 4,946,891. That process describes the polymerization of vinyl acetate with other liquid monomers such as butyl acrylate. The object was to reduce the particle size of the emulsion polymers as compared with fully continuous operation, and to improve properties such as gloss and wet abrasion of emulsion paints. The polymerization is carried out under atmospheric pressure and the product is withdrawn by simple overflow of the reactor into a second vessel.
EP 1266909 B1 describes a semicontinuous process for emulsion polymerization for producing a bimodal product. In that process, the reactants are metered into a reactor and the batch is brought to polymerization temperature. Then surface-active agent or seed latex is added. When the reactor is full, product is withdrawn continuously from the reactor, at the same rate as that of the metered feed, and is collected in a second container. Operation is continued until the volume metered in the reactor is 1.05 to 2 times the reactor volume.